Light to frequency converters (LFCs) are used to measure the intensity of light. Generally, such optical sensors consist of an electronic circuit having a photo detector and a current (or voltage) to frequency converter. FIG. 1 is a block diagram of a typical light to frequency converter. The photo detector produces a voltage or a current. The current (or voltage) to frequency converter produces a frequency that generally is proportional to the light intensity detected or received by the photo detector. As illustrated in FIG. 1, light input 2 couples received light to photo detector 4, which is coupled via line 6 to current to frequency converter 8, which outputs on line 10 a signal having a frequency that is proportional to the light intensity detected by photo detector 4.
Typical photo detectors are photo-transistors or photo diodes that are implemented with a suitable power source to produce a current, or photo cells that produce both a current and a voltage. In either case, whether the photo detector produces a voltage or current the principals of operation of the light to frequency converter in general are the same.
FIG. 2 is a typical current to frequency converter. As illustrated, it consists of integrator 14 that integrates the input current on line 12 (or voltage used to produce a current), threshold detector comparator 18 coupled to integrator 14 that detects when integrator 14 reaches a threshold level and a discharge circuit (preferably internal to integrator 14 and not separately shown) coupled via line 20 that resets integrator 14 when the integrator output reaches the threshold level. The frequency of the pulses produced by threshold detector comparator 18 (LFC output) on output line 22 in general are proportional to the intensity of the input current on line 12. As the current is increased the output frequency is increased.
Light to frequency converters have been commercially available for several decades. Two suppliers are Texas Advanced Optical Systems and Hamamatsu.
The inventors have disclosed methods for measuring the output of light to frequency converters (see, e.g., U.S. application Ser. No. 09/091,208, filed on Jun. 8, 1998, which is based on International Application No. PCT/US97/00126, filed on Jan. 2, 1997, which is a continuation in part of U.S. application Ser. No. 08/581,851, now U.S. Pat. No. 5,745,229, issued Apr. 28, 1998, for Apparatus and Method for Measuring Optical Characteristics of an Object; U.S. application Ser. No. 9/091,170, filed on Jun. 8, 1998, which is based on International Application No. PCT/US97/00129, filed on Jan. 2, 1997, which is a continuation in part of U.S. application Ser. No. 08/582,054, now U.S. Pat. No. 5,759,030 issued Jun. 2, 1998,for Apparatus and Method for Measuring Optical Characteristics of Teeth; PCT Application No. PCT/US98/13764, filed on Jun. 30, 1998, which is a continuation in part of U.S. application Ser. No. 08/886,223, filed on Jul. 1, 1997, for Apparatus and Method for Measuring Optical Characteristics of an Object; PCT Application No. PCT/US98/13765, filed on Jun. 30, 1998, which is a continuation in part of U.S. application Ser. No. 08/886,564, filed on Jun. 30, 1998, for Apparatus and Method for Measuring Optical Characteristics of Teeth; and U.S. application Ser. No. 08/886,566, filed on Jul. 1, 1997, for Method and Apparatus for Detecting and Preventing Counterfeiting. Reference is also made to PCT App. Ser. No. PCT/US03/05310 filed on Feb. 21, 2003, which is a continuation in part of U.S. App. Ser. No. 10/081,879, filed on Feb. 21, 2002; the foregoing patent documents are sometimes referenced collectively herein as the “Referenced Patent Documents” and are incorporated herein by reference for their use of LFCs in various circuits, systems, methods and applications).
One method is to count the number of output pulses for a fixed period of time. The number of pulses is proportional to the light intensity. Unfortunately, this method produces low grayscale photonic resolution when light intensities are low because the intensity range may only produce a limited number of pulses. When a LFC is utilized in a spectroscopy application where multiple LFCs receive a limited range of light wavelengths, the intensity is further reduced. Another disadvantage of counting the output pulses is that there is a minimum light intensity threshold needed to produce a single pulse in the measurement light period. If the intensity is too low, an intensity measurement cannot be made.
As disclosed in the Referenced Patent Documents, a preferred method to measure the output of an LFC is to both count the number of output pulses for a fixed time period and also measure the amount of time that elapsed between the first pulse and last pulse in the measurement time period. The intensity of the LFC is then calculated by dividing the number of pulses by time elapsed between the first and last pulse and reporting the quotient as a floating point number. As long as there are two or more pulses the light intensity can be measured and the grayscale photonic resolution is independent of light intensity. The grayscale resolution is determined by the resolution of the timing clock measuring the time between the first and last pulse.
1. LFC Intensity (Number of Pulses)/(Time Between First and Last Pulse)
However, there is a minimum intensity required to produce a measurement since at least two pulses are needed to determine the time between the first and last pulse. As disclosed in the Referenced Patent Documents, in one preferred arrangement the LFC can be biased by a stable light source to ensure a minimum threshold is present. When the sample light source is applied, the output of the LFC will be the sum of the bias intensity and the light source intensity. As long as the bias light source is stable (or otherwise monitored) it can be subtracted from the LFC output to determine the intensity of the unknown light source.
2. Intensity=Intensity of Sample−Intensity of Bias
Providing a bias light source increase circuit complexity and costs. Accordingly, there is a need for improved optical sensors, and more particularly there is a need for improved light to frequency converter optical sensor-based systems and methods utilizing electronic bias and adjustable gain in such systems.